Definify.com

Webster 1913 Edition

Noun.

[L.

tetras

-adis

, Gr. [GREEK], [GREEK]: cf. F.

tétrade

The number four; a collection of four things; a quaternion.

(Chem.)

A tetravalent or quadrivalent atom or radical;

as, carbon is a

tetrad

Noun.

[Gr. the number four.] The number four; a collection of four things.

tetrad ‎(plural tetrads)

A group of four things.
(cartography) A unit of land area of two by two (that is, four) square kilometres.
- 2010, Richard Mabey, Weeds: How Vagabond Plants Gatecrashed Civilisation and Changed the Way We Think about Nature, London: Profile Books, ISBN 978-1-84668-076-2:
  They took figures for the abundance of invasive weeds mapped according to the normal grid unit of the 'hectad', or 10 × 10 km square, and then looked at how abundant these species were mapped at a much finer scale – in 'tetrads', or 2 × 2 km squares, inside these hectads.
(chemistry) A tetravalent atom or radical.
(biology) Two pairs of sister chromatids (a dyad pair) aligned in a certain way and often on the equatorial plane during the meiosis process.
(biology) A group of four haploid and immature pollen grains in tetrahedral fashion produced by meiotic microsporogenesis.